In the thirty years leading up to the Civil War, tensions in the country mounted over the issue of slavery. By 1830, there were more than 2 million slaves in the United States, worth over a billion dollars (compared to annual federal revenues of less than 25 million). And their numbers were growing. During the 1830s alone, the migration of slaves to the lower South increased the slave population in Georgia, Mississippi, Alabama, Louisiana, Florida and Arkansas from 530,404 to 943,881.

Even with this enormous expansion of slavery, 75 percent of white southerners did not own slaves. Of those who did, the vast majority owned no more than 20. The bulk of the enormous wealth produced by slave-grown cotton rested in the hands of a few planters. A significant portion of the Northern industrial economy rested on slave-grown cotton as well, and this contributed to northerners' hostility to the abolitionist movement.

The abolitionists gained momentum in this period, however. Freed and escaped slaves spread their stories through publications and speeches at local and national antislavery meetings. As the country expanded westward to Kansas and Nebraska, Texas, New Mexico and California, pushing out Native Americans and Mexicans, the question of whether slavery should exist in the new territories fueled the growing divisions in the nation.

Strong differences of opinion over the slavery question led to violent clashes, culminating in the raid on Harpers Ferry. After the election of President Lincoln in 1860, seven southern states seceded and Civil War broke out, followed by four years of bloody fighting and the loss of 617,000 American lives. The Union survived, however, and with the end of the war in 1865, long after the Emancipation Proclamation had pronounced slaves in seceded lands free, all African Americans finally emerged from their 250 years of bondage into their freedom as citizens of the United States of America.

1845: Slaveholding Texas is annexed into the United States, sparking the Mexican War. By 1848, Mexico cedes more than half of its territory to the U.S., including New Mexico and California in addition to Texas.

1850: Its population swelled by the recent Gold Rush, California enters the Union as a free state. In return, slaveholders in the South are given a stringent Fugitive Slave Law for the recapture of runaways.